One type of liquid crystal light valve is a projection-type image display apparatus. The light valve comprises an evacuated ceramic envelope that incorporates a pair of opposing transparent windows as part of the envelope wall. One window forms part of a liquid crystal cell. The remainder of the cell is assembled adjacent to the inner surface of that window. Polarized light is typically directed through the envelope windows. The light exiting the liquid crystal cell passes through a polarizing filter or analyzer. Any light passing through the analyzer is transmitted via a projection lens system onto a suitable viewing surface.
The liquid crystal cell includes liquid crystal material captured between two substrates. One substrate is formed of glass and is incorporated into the wall of the envelope to serve as a window as mentioned above. The opposing substrate comprises a thin, dielectric material. The cell is preferably of the twisted nematic type, which is constructed so that in the absence of an applied electric field (i.e., with the cell in the "OFF" state) the cell rotates by 90.degree. the polarization direction of the projected light. With the cell in the "OFF" state, no light passes through the analyzer and the viewing surface remains dark. When an electric field is applied to the cell (i.e.. when the cell is switched to the "ON" state) the projected light passes through the cell with the polarization direction unchanged. As a result, the light passes through the analyzer to the viewing surface.
An electron beam-addressed liquid crystal light valve employs an electron beam for modulating the polarization direction of light propagating through the liquid crystal cell. Specifically, an electron gun is mounted within the light valve envelope and provides a beam of electrons that strike the side of the liquid crystal cell carrying the thin dielectric substrate, which is called the target substrate. The electron beam is deflected by suitable circuitry to raster scan the target substrate. A collector electrode is mounted within the liquid crystal light valve near the target substrate. To produce or "write" an image, the electron beam and collector electrode positioned near the target substrate cooperate to develop an electrostatic potential at certain points on the surface of the target substrate that correspond to the desired image. As a result, those points on the liquid crystal cell are switched to the "ON" state, thereby permitting associated portions of the projected light of unchanged polarization direction to pass through the valve and form the image on the viewing surface.
Copending U.S. patent application Ser. No. 046,822 of Buzak et al., entitled Liquid Crystal Light Valve with Electrically Switchable Secondary Electron Collector Electrode, filed concurrently herewith, describes a particular configuration and optimal operation mode for an electron beam-addressed liquid crystal light valve. More particularly, that application describes a write mode wherein the collector electrode is maintained at positive voltage relative to the target substrate. The electron beam bombards the target substrate with electrons of sufficient energy to result in secondary electron emissions from the target substrate surface. The secondary electrons are collected on the positively charged collector electrode. The rate of secondary electron emission is greater than the rate at which the incident electrons are delivered by the electron gun. Consequently, the beam-addressed area of the target substrate surface is driven to a positive potential. This change in potential switches the corresponding region of the liquid crystal cell to the "ON" state.
To erase the image (i.e., to switch the previously written region of the liquid crystal cell to the "OFF" state), the collector electrode is switched to a negative potential relative to the written region of the target substrate. An electron gun (either the writing gun or a separate gun) scans an electron beam over the target substrate. The resulting secondary electrons, repelled by the negatively charged collector electrode, are directed to the previously written (positively charged) regions. Accordingly, the positive potential at the previously written areas of the target substrate is removed and the corresponding region of the liquid crystal cell is switched to the "OFF" state.
The target substrate for an electron beam-addressed liquid crystal light valve as just described is preferably formed of thin dielectric material having secondary emission characteristics that permit the electron beam to be driven with relatively low electrical current. The dielectric target substrate must be thin to minimize spreading of the electric field lines produced by the charge deposited on the surface. Spreading of the electric field lines reduces the resolution of the projected image. Driving the electron beam with relatively low electrical current results in a smaller beam spot size and a correspondingly higher resolution of the produced image. A mica film of five to ten microns in thickness forms a suitable target substrate.
Prior to assembly of the liquid crystal cell, it is necessary to treat the surfaces of the cell substrates that contact the nematic material. The surfaces are treated to provide a homogeneous (i.e., parallel) surface alignment of the nematic liquid crystal material. Typically, the surface treatment is provided by vacuum-depositing silicon monoxide onto the substrate at an angle of approximately 5.degree. relative to the plane of the substrate surface. Effective treatment of the thin flexible substrate requires that the substrate remain planar during and after the deposition process so that no irregularities occur in the silicon monoxide layer. Further, the substrate must be supported in the planar orientation without the use of clamping devices that might damage the substrate.
For proper operation of the assembled liquid crystal cell, it is important that spacing between the target substrate and the rigid glass substrate be maintained with great precision so that the thickness of the liquid crystal material is uniform throughout the cell. When a thin dielectric target substrate such as mica is used in a liquid crystal light valve, establishing precisely uniform spacing between the substrates may be difficult. The difficulty results from the flexible nature of the target substrate, which provides little spacing control. Using spacers such as numerous glass spheres distributed between the substrates provides some control; however, these spacers do not prevent the flexible target substrate from bending away from the rigid substrate or toward the rigid substrate between the spacers.